This invention relates to a process for producing an oligoester which is an intermediate for preparing unsaturated polyester resin from isophthalic acid and propylene oxide, and also relates to a process for producing unsaturated polyester resin from the oligoester and unsaturated dibasic acid or its anhydride.
In the present specification "unsaturated polyester resin" indicates the polyester resin which is prepared from dibasic acid components containing unsaturated dibasic acid and polyol components and is not yet mixed with a vinyl monomer as a crosslinking agent. The mixture of the unsaturated polyester resin and a vinyl monomer is referred to as "liquid unsaturated polyester resin" in the present specification.
It is known that unsaturated polyester resins obtained by using isophthalic acid as a dibasic acid component (sometimes hereinunder referred to as isophthalic acid type unsaturated polyester resin) are superior to unsaturated polyester resins obtained by using ortho-phthalic acid as a dibasic acid component with respect to such properties as water resistance, chemical resistance and mechanical properties. However, the time required for esterification is longer when isophthalic acid is used as a dibasic acid component than when ortho-phthalic acid is used.
The processes for producing isophthalic acid type unsaturated polyester resins are of two types; one stage and two stage. In the one stage, all of the dibasic acid components containing isophthalic acid and unsaturated dibasic acid are reacted with all of the polyol component, such as glycol, simultaneously. The two stage process comprises reacting isophthalic acid with a polyol component, such as a glycol, until the acid value amounts to less than a definite value, for example less than 30, to form an oligoester, followed by reacting unsaturated dibasic acid or its anhydride with the resulting oligoester which is a precursor for unsaturated polyester resin. Though the reaction time of the one stage process is shorter than that of the two stage process, it is known that unsaturated polyester resin produced by the one stage process are inferior to unsaturated polyester resins produced by the two stage process with respect to various properties. (Refer to "Reinforced Plastics," vol. 19, 1973 page 38.) For example unsaturated polyester resins obtained by the one stage process have low water resistance, chemical resistance and mechanical properties. So, to obtain unsaturated polyester resins with suitable characteristics the two stage process has been desired.
Many processes for producing unsaturated polyester resins by reacting an alkylene oxide with dicarboxylic acid or its anhydride have been proposed, but a process for producing unsaturated polyester resins by using isophthalic acid as a dibasic acid component and an alkylene oxide has not been utilized in the art. The reason is that the melting point of isophthalic acid is very high, so its solubility in a reaction medium is too low to allow the reaction to proceed.
The addition product of an aromatic dicarboxylic acid and an alkylene oxide can be easily produced by reacting the two components in the presence of an amine compound catalyst, such as a tertiary amine, so it was known that the addition product is utilized as an intermediate for saturated linear polyester, which is in general used for fibers and molding materials.
On the other hand, when the addition product is utilized as an intermediate for unsaturated polyester resin, the resulting resin is apt to be seriously discolored, and is limited to reproducibility of various characteristics with reference to the curing step of liquid unsaturated polyester resin and various properties of the cured resin, such as mechanical properties, chemical resistance, water resistance and so on, and additionally it has low storage stability. So the resin is not of practical use.
Reacting an aromatic dicarboxylic acid with an alkylene oxide in the absence of any catalyst has been attempted in order to overcome the above mentioned shortcoming. In this case the addition reaction between isophthalic acid and an alkylene oxide is hard to realize because of the high melting point of isophthalic acid and low solubility of isophthalic acid in the reaction medium and simultaneously polymerization of the alkylene oxide itself occurs to considerable extent. In addition, it takes much time to lower the acid value of the oligoester to the necessary extend. So, it has been believed to be impossible to economically obtain unsaturated polyester resins with suitable characteristics by reacting an isophthalic acid with an alkylene oxide.
Furthermore, when the oligoester of an isophthalic acid and an alkylene oxide is used as an intermediate for unsaturated polyester resins, whether the oligoester can fill requirements for various compositions of unsaturated polyester resins is practically important. Many attempts have been directed to use of an addition product of aromatic dibasic acid and an alkylene oxide as an intermediate for polyester resins, wherein the molar ratio of alkylene oxide to aromatic dibasic acid was approximately 2 as usual. Accordingly the addition product of isophthalic acid and an alkylene oxide produced by the prior art is usable only for the specific composition of unsaturated polyester resin, but is not suitable for various compositions thereof.